ACCUMULO-1833 Another instance of primitive without synchronization being used instead of
AtomicBoolean with expected concurrent access.
ACCUMULO-1833 Update MTBW close method to match what TSBW is doing
(update internal boolean then perform the close)
ACCUMULO-1833 Ensure that we close the MTBW at the end of the test to
avoid it getting GC'ed later and trying to flush when ZK and the
instance is already gone.